The present invention relates to a process for producing a bismuth-based superconducting oxide (hereinafter referred to as Bi-based superconducting oxide). More particularly, the present invention relates to a process for producing a Bi-based superconducting oxide, which comprises subjecting a starting material for Bi-based superconducting oxide to partial melting and then to crystallization to obtain a Bi-based superconducting oxide.
In recent years, by using a superconducting magnet made of a superconducting oxide having superconducting properties, there have been or are being developed various apparatuses or techniques such as nuclear magnetic resonance (NMR) analyzer, NMR tomograph, levitated train and the like. Further, it is being investigated to apply the strong magnetism of superconducting magnet to development of new energy (e.g. fusion reactor) or new energy conversion techniques (e.g. magnethydrodynamics power generation).
As such a superconducting oxide, there have been studied and developed various superconducting oxides. Of these superconducting materials, Bi-based superconducting oxides of Bi-Sr-Ca-Cu-O type have high critical temperatures (Tc) and are drawing attention.
The Bi-based superconducting oxides are generally produced by mixing starting materials to obtain a starting material powder so as to give a composition showing superconducting properties, molding the powder and firing the molded article.
In order for a sintered article to have superconducting properties and a practical critical current density (J.sub.c), it is necessary that the sintered article contain no impurities and the particles in the sintered article are bonded with each other so as to allow the smooth flow of supercurrent.
In the above production process, however, the impurities contained in the starting materials are not decomposed thoroughly and part of them remains in the final sintered article, and although the particles in the sintered article are bonded, the crystal faces are nonuniform and irregular; therefore, the above process is insufficient to obtain a high J.sub.c.
There is also proposed a process wherein a starting oxide for Bi-based superconducting material is, after melting, cooled slowly and then crystallized in order to obtain a higher crystallinity. In this process, however, crystal phase other than desired are present together and accordingly no crystal phase of desired composition is formed, making it impossible to obtain a high J.sub.c. Further in the process, the time for crystal growth is too long.
Further, a superconducting sintered article causing substantially no shrinkage can be obtained by subjecting a starting material powder for Bi-based superconducting oxide to molding and firing and, in that case, effecting the firing for a long period of time of about 1 month at the melting point of the superconducting oxide or below. However, the J.sub.c of the article is not high and accordingly the article is not satisfactory as a superconducting oxide.
Therefore, the first object of the present invention is to provide a process for producing a Bi-based superconducting oxide superior in superconducting properties (e.g. J.sub.c).
The above and other objects of the present invention will become apparent from the following description.